Seals are small stickers introduced in Generation IV that are able to be attached to Ball Capsules, which can in turn be attached to Poké Balls, to attain a special effect when a Pokémon comes out of them, instead of the usual flash of light. In Pokémon Diamond, Pearl, and Platinum, a majority of Seals can be purchased at the Market in Sunyshore City, where the selection changes daily. However, there are an additional 28 Seals (letters A through Z, an exclamation point and a question mark), obtained by showing Unown to a boy in Solaceon Town. This can be repeated as often as needed - a new Unown is not needed each time. In HeartGold and SoulSilver, three Seals can be acquired each day from a girl in Olivine City. Once a Seal is bought, it cannot be sold or thrown away legitimately.

Eight seals at a time can be attached to any Ball Capsule. They can be set in the player's PC. Where they are placed on the Ball Capsule will affect where the effect is displayed upon the Pokémon's release.

In the anime

The very first episode that featured the use of Seals in the Anime was in A Friend In Deed by Ritchie, although these were just non-functioning Star Stickers that he applied directly on his Poké Balls to distinguish them from the rest of the ones stolen by Team Rocket and that the concept and use of Seals and Ball Capsules did not exist (outside of Sinnoh) at the time.
Seals are commonly used in contests. These make the entrance of the Pokémon more attractive and may sometimes affect the score given to them also. A few Seals are supplied when a coordinator receives a Seal Case, but the majority are bought at shops and markets.